As a cooling device for an internal combustion engine, there is known an ebullient cooling device that cools it by utilizing boiling evaporation heat of a coolant flowing through a coolant passage (for example, a water jacket) formed within the internal combustion engine. The ebullient cooling device has the coolant passage that is connected to, for example, a gas-liquid separator. The gas-liquid separator separates the coolant discharged from the coolant passage into a liquid-phase coolant and a gas-phase coolant. Additionally, there is known the ebullient cooling device that supplies the liquid-phase coolant to the coolant passage from a condenser through the gas-liquid separator when the cooling of the internal combustion engine is insufficient (for example, see Patent Documents 1 and 2).